Present television cameras produce an optical image of an object on the imaging area of a charge integration type image pickup tube or a solid-state imaging device and scan the signal charge developed on this imaging area at a predetermined frequency to convert it to an electrical signal. This scanning frequency is predetermined in accordance with the television system employed, and is 60 fields/sec in the NTSC system, for example. Therefore, the signal storage time is 16.7 msec (in the case of signal readout with field storage) or 33.3 msec (in the case of signal readout with frame storage).
Solid-state imaging devices put into practical application as television cameras of the kind described above include an MOS type device and a CCD type device. These devices convert the optical image to signal charges using photoelements (e.g. photo-diodes) formed on a semiconductor substrate, store the signal charges, read out the signal charges at a predetermined frequency by a scanning circuit or the like and output video signals. In the charge integration type solid-state imaging device which stores the signal charges in the parasitic capacity of the photoelement (photo-diode), the sum of signal charges that can be stored (saturated signal charge) is determined to a predetermined value by the parasitic capacity of each photoelement.
On the other hand, the quantity of signal charge stored in each photoelement is proportional to the product of a signal storage time of each photoelement and the intensity of rays of light incident on each photoelement (or scene illuminance). Therefore, the quantity of signal charge stored in the photoelement becomes great when the signal storage time is long or when the object is bright and the intensity of incident rays is high, and becomes small when the signal storage time is short or when the object is dark and the intensity of incident rays of light is low, on the contrary. Since substantially constant noise charge mixes in the signal charge read out from the photoelement, the signal-to-noise ratio (S/N) of the imaging device is proportional to the quantity of signal charge stored in the photoelement. In other words, S/N changes depending on the signal storage time and the intensity of incident rays (scene illuminance). For instance, S/N can be improved by about 6 dB if the signal storage time is doubled or the intensity of incident rays is doubled.
In order to improve the S/N of the imaging device on the basis of the principle described above, the following prior art references attempt to control the signal storage time in accordance with the intensity of incident rays (scene illuminance):
(a) Japanese Patent Laid-Open No. 140,510/1976 PA1 (b) Japanese Patent Laid-Open No. 110,209/1980 PA1 (c) Japanese Patent Laid-Open No. 64,711/1982 PA1 (d) Japanese Patent Laid-Open No. 196,666/1984 PA1 (e) Japanese Patent Laid-Open No. 74,878/1985
These prior art references relate to a charge integration type photodetector used in an automatic focus detector of a camera or the like, and can set arbitrarily the storage time of the signal charge in accordance with the intensity of incident rays (scene illuminance). However, in the television camera to which the present invention is directed, the scanning frequency is fixed to 60 fields/sec. Therefore, the maximum storage time of the signal charge is 1/60 sec (16.7 msec) in the system employing signal readout with field storage and at most 1/30 sec (33.3 msec) in the system employing signal readout with frame storage. For this reason, when an object having low illuminance with low intensity of incident rays is imaged, the maximum storage time is fixed to either 1/60 sec (16.7 msec) or 1/30 sec (33.3 msec) even when one attempts to extend the storage time in order to improve S/N. Accordingly, degradation of image quality due to the drop of S/N is unavoidable.
Moreover, the prior art technique described above controls the storage time only through the intensity of incident rays (scene illuminance) but does not at all take moving objects into consideration. In other words, these references do not mention at all the drop (obscurity of image) of dynamic resolution of moving pictures that occurs with the increase in the storage time. Disclosure of Invention:
It is therefore an object of the present invention to provide a solid-state television camera which is devoid of the drop of dynamic resolution for moving objects and provides high S/N for still objects.
The solid-state television camera in accordance with the present invention includes a first group of photoelements for storing the signal charge in accordance with scene illuminance, a second group of photoelements for storing the signal charge in accordance with the scene illuminance, motion detection means for detecting the motion of an object on the basis of the signals from said first group, and storage time control means for controlling the storage time (scanning frequency) of said second group on the basis of the signals from the motion detection means.
In accordance with the present invention, the drop of dynamic resolution is unlikely to occur in motion pictures because the storage time is short, and S/N becomes high in still pictures because the storage time becomes long. Thus, the invention can effectively control dynamic resolution as well as S/N and can obtain video signals having high picture quality.